In RF vacuum microwave devices such as klystrons, an electron beam is manipulated and its kinetic energy is partially converted into RF energy. This process is not fully efficient, and the depleted beam is collected by a collector. Conventionally, the energy deposited into the collector is lost as heat. In pulsed systems, the energy during the rise and fall of a driving pulse (from a modulator) is entirely lost as heat. RF energy is removed only during the flat top portion of the pulse.
There are typically three methods utilized to improve this efficiency. First, the beam to RF conversion efficiency is studied. Research in this area is ongoing, but it requires fundamental changes to the tube technology and potentially has impacts on tube performance. Second, the rise and fall times to the klystron are shortened. This is a very hard parameter to improve upon, especially for high power tubes. Third, a device called a depressed collector can be used.
Depressed collectors in RF amplifiers are a mature and successful technology for efficiency-critical applications such as space applications and UHF broadcast. They are typically employed in low power CW tubes and function by extracting energy from the spent electron beam, shown in FIG. 1. In this example, a spent beam 116 from a klystron device 100 having an output cavity 114 first passes through a reconditioner 102 and then into a depressed collector 118 having five conductive stages 104, 106, 108, 110, 112 that are biased at negative electrical potentials below the kinetic energy of the beam 116. As the beam travels through the depressed collector 118, the electron momentum decreases until collected by a stage. Ideally, the momentum is reduced to zero just as it impacts a collector stage. As less work is required to stop the electrons, this stage biasing and collection of electrons results in reduced heat dissipation in the collector. Effectively, depending upon the stage biasing topology, a portion of the beam power is recovered to some point within the driving modulator/power supply, resulting in a reduced AC power draw. Reduced power draw with the same RF power out results in a higher system efficiency.
The present state-of-the-art in multi-stage collectors can only efficiently recover energy in CW systems. The energy in the rise and fall of pulses is lost. In addition, in conventional depressed collectors, the power supplies are arranged in discharge mode which necessitates driving them to particular potentials.
Many accelerator applications utilize pulsed, high peak power RF systems with duty cycles less than 1%. Typically, a high voltage, pulsed modulator delivers a pulse to the cathode of a klystron. A pulse shape, with rise, flattop, and fall times can be defined as in FIG. 2. Because accelerator applications require high RF phase stability during the pulse, the low level RF input is only applied during the high voltage modulator pulse flat top. Therefore, all of the beam energy during the modulator rise and fall times is wasted and is dissipated as heat in the klystron collector. The amount of energy wasted in the collector is significant for short pulse, low duty cycle systems. For very short pulse applications, this problem is compounded: fast rise and fall times are very hard to achieve in high power modulators.
With a pulsed, high-power system, utilization of a depressed collector to increase system efficiency is not straightforward. The conventional method for applying a pulsed potential to a collector stage is to tap off of the secondary of the output transformer of the modulator. While appropriate for some applications, this approach is not viable for many accelerator applications: it causes deleterious cathode voltage ringing due to parasitic impedances. This ringing translates into RF phase jitter and unacceptable performance.